Re: Help with join syntax sought

Andy Colson wrote:
> James B. Byrne wrote:
>> I have a requirement to select the effective exchange rate for a
>> number of currencies as of a specific date and time. The rates may
>> come from several sources for the same currency. For some
>> currencies the rate may be set infrequently. I have come close to
>> getting this to work but cannot seem to get the last bit figured
>> out. Thus my appeal for help.
>>
>>
>> Here is what I have so far:
>>
>> SELECT
>> fxr.currency_code_base AS fx_base,
>> fxr.currency_code_quote AS fx_quote,
>> fxr.effective_from AS fx_date,
>> fxr.currency_exchange_type AS fx_type,
>> fxr.currency_exchange_rate AS fx_rate
>>
>> FROM
>> currency_exchange_rates AS fxr
>>
>> LEFT OUTER JOIN
>> currency_exchange_rates AS fxr_j
>>
>> ON
>> fxr.currency_code_base = fxr_j.currency_code_base
>> AND
>> fxr.currency_code_quote = fxr_j.currency_code_quote
>> AND
>> fxr.currency_exchange_type = fxr_j.currency_exchange_type
>> AND
>> fxr.effective_from >= fxr_j.effective_from
>>
>> WHERE
>> fxr.currency_code_base = 'CAD'
>> AND
>> fxr.effective_from <= current_timestamp
>>
>> GROUP BY
>> fx_base,
>> fxr.currency_code_quote,
>> fx_date,
>> fxr.currency_exchange_type,
>> fx_rate
>>
>> HAVING
>> COUNT(fxr.currency_code_quote) = 1
>>
>> ORDER BY
>> fx_base,
>> fxr.currency_code_quote,
>> fx_date DESC
>>
>>
>
> I see currency_code_base = 'CAD', so you are looking for the most recent
> Canadian exchange rate.
>
>
> > The rates may
> > come from several sources for the same currency.
>
> What field is the source? currency_code_quote?
>
> -Andy

Hum... I can quite figure what we are looking for.

you say: currencies as of a specific date and time
but there is not date stuff in the where... so we'll ignore that.

you say: come from several sources
but doesnt seem important, we'll ignore that.

you say: I want only the most recent regardless of type
so type is unimporttant

How about this:

select * from fxr where code_base = 'CAD' and effective_from = (select
max(effective_from) from fxr f2 where f2.code_base=fxr.code_base and
f2.code_quote=fxr.code_quote);

(forgive the shortened names), it selects any 'CAD' of only the most
recent time, based on code_base and code_quote. (had to guess at the
fields)

Oh! I see one problem.. if the effective_from is exactly the same it
could return more records than you want. Have to think about that...

-Andy